Thanks for all comments and responses on previous page.
This problem has caused me to do some more careful testing over recent days, and to especially concentrate on wots 'appening.
I've realised that when both the radio and navigation are in concurrent use, the volume knob and steering wheel controls do indeed control the volume of the radio, AND the navigation instructions, independently.
When the radio sound is playing solely, the volume controls set the radio volume.
When the navigation sound plays over the top of the radio, the volume controls set the navigation volume.
So you need to be alert to what sound is 'appening, and be quick to use the volume controls at the right moments.
With some small practice, this seems to be a perfectly simple and easy solution. Some might say logical too.
And its easy to mute the radio, whilst keeping the navigation directions at the correct sound level.
Sorry it took me a while to get with the program, but I hope this description helps somebuddy else too.

How can I mark this thread title [SOLVED], or can a mod do that ?